xtensa: support hardware breakpoints/watchpoints

Use perf framework to manage hardware instruction and data breakpoints.
Add two new ptrace calls: PTRACE_GETHBPREGS and PTRACE_SETHBPREGS to
query and set instruction and data breakpoints.
Address bit 0 choose instruction (0) or data (1) break register, bits
31..1 are the register number.
Both calls transfer two 32-bit words: address (0) and control (1).
Instruction breakpoint contorl word is 0 to clear breakpoint, 1 to set.
Data breakpoint control word bit 31 is 'trigger on store', bit 30 is
'trigger on load, bits 29..0 are length. Length 0 is used to clear a
breakpoint. To set a breakpoint length must be a power of 2 in the range
1..64 and the address must be length-aligned.

Introduce new thread_info flag: TIF_DB_DISABLED. Set it if debug
exception is raised by the kernel code accessing watched userspace
address and disable corresponding data breakpoint. On exit to userspace
check that flag and, if set, restore all data breakpoints.

Handle debug exceptions raised with PS.EXCM set. This may happen when
window overflow/underflow handler or fast exception handler hits data
breakpoint, in which case save and disable all data breakpoints,
single-step faulting instruction and restore data breakpoints.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

1 files changed, 317 insertions, 0 deletions

diff --git a/arch/xtensa/kernel/hw_breakpoint.c b/arch/xtensa/kernel/hw_breakpoint.c
new file mode 100644
index 000000000000..b35656ab7dbd
--- /dev/null
+++ b/arch/xtensa/kernel/hw_breakpoint.c
@@ -0,0 +1,317 @@
+/*
+ * Xtensa hardware breakpoints/watchpoints handling functions
+ *
+ * This file is subject to the terms and conditions of the GNU General Public
+ * License.  See the file "COPYING" in the main directory of this archive
+ * for more details.
+ *
+ * Copyright (C) 2016 Cadence Design Systems Inc.
+ */
+
+#include <linux/hw_breakpoint.h>
+#include <linux/log2.h>
+#include <linux/percpu.h>
+#include <linux/perf_event.h>
+#include <variant/core.h>
+
+/* Breakpoint currently in use for each IBREAKA. */
+static DEFINE_PER_CPU(struct perf_event *, bp_on_reg[XCHAL_NUM_IBREAK]);
+
+/* Watchpoint currently in use for each DBREAKA. */
+static DEFINE_PER_CPU(struct perf_event *, wp_on_reg[XCHAL_NUM_DBREAK]);
+
+int hw_breakpoint_slots(int type)
+{
+	switch (type) {
+	case TYPE_INST:
+		return XCHAL_NUM_IBREAK;
+	case TYPE_DATA:
+		return XCHAL_NUM_DBREAK;
+	default:
+		pr_warn("unknown slot type: %d\n", type);
+		return 0;
+	}
+}
+
+int arch_check_bp_in_kernelspace(struct perf_event *bp)
+{
+	unsigned int len;
+	unsigned long va;
+	struct arch_hw_breakpoint *info = counter_arch_bp(bp);
+
+	va = info->address;
+	len = bp->attr.bp_len;
+
+	return (va >= TASK_SIZE) && ((va + len - 1) >= TASK_SIZE);
+}
+
+/*
+ * Construct an arch_hw_breakpoint from a perf_event.
+ */
+static int arch_build_bp_info(struct perf_event *bp)
+{
+	struct arch_hw_breakpoint *info = counter_arch_bp(bp);
+
+	/* Type */
+	switch (bp->attr.bp_type) {
+	case HW_BREAKPOINT_X:
+		info->type = XTENSA_BREAKPOINT_EXECUTE;
+		break;
+	case HW_BREAKPOINT_R:
+		info->type = XTENSA_BREAKPOINT_LOAD;
+		break;
+	case HW_BREAKPOINT_W:
+		info->type = XTENSA_BREAKPOINT_STORE;
+		break;
+	case HW_BREAKPOINT_RW:
+		info->type = XTENSA_BREAKPOINT_LOAD | XTENSA_BREAKPOINT_STORE;
+		break;
+	default:
+		return -EINVAL;
+	}
+
+	/* Len */
+	info->len = bp->attr.bp_len;
+	if (info->len < 1 || info->len > 64 || !is_power_of_2(info->len))
+		return -EINVAL;
+
+	/* Address */
+	info->address = bp->attr.bp_addr;
+	if (info->address & (info->len - 1))
+		return -EINVAL;
+
+	return 0;
+}
+
+int arch_validate_hwbkpt_settings(struct perf_event *bp)
+{
+	int ret;
+
+	/* Build the arch_hw_breakpoint. */
+	ret = arch_build_bp_info(bp);
+	return ret;
+}
+
+int hw_breakpoint_exceptions_notify(struct notifier_block *unused,
+				    unsigned long val, void *data)
+{
+	return NOTIFY_DONE;
+}
+
+static void xtensa_wsr(unsigned long v, u8 sr)
+{
+	/* We don't have indexed wsr and creating instruction dynamically
+	 * doesn't seem worth it given how small XCHAL_NUM_IBREAK and
+	 * XCHAL_NUM_DBREAK are. Thus the switch. In case build breaks here
+	 * the switch below needs to be extended.
+	 */
+	BUILD_BUG_ON(XCHAL_NUM_IBREAK > 2);
+	BUILD_BUG_ON(XCHAL_NUM_DBREAK > 2);
+
+	switch (sr) {
+#if XCHAL_NUM_IBREAK > 0
+	case SREG_IBREAKA + 0:
+		WSR(v, SREG_IBREAKA + 0);
+		break;
+#endif
+#if XCHAL_NUM_IBREAK > 1
+	case SREG_IBREAKA + 1:
+		WSR(v, SREG_IBREAKA + 1);
+		break;
+#endif
+
+#if XCHAL_NUM_DBREAK > 0
+	case SREG_DBREAKA + 0:
+		WSR(v, SREG_DBREAKA + 0);
+		break;
+	case SREG_DBREAKC + 0:
+		WSR(v, SREG_DBREAKC + 0);
+		break;
+#endif
+#if XCHAL_NUM_DBREAK > 1
+	case SREG_DBREAKA + 1:
+		WSR(v, SREG_DBREAKA + 1);
+		break;
+
+	case SREG_DBREAKC + 1:
+		WSR(v, SREG_DBREAKC + 1);
+		break;
+#endif
+	}
+}
+
+static int alloc_slot(struct perf_event **slot, size_t n,
+		      struct perf_event *bp)
+{
+	size_t i;
+
+	for (i = 0; i < n; ++i) {
+		if (!slot[i]) {
+			slot[i] = bp;
+			return i;
+		}
+	}
+	return -EBUSY;
+}
+
+static void set_ibreak_regs(int reg, struct perf_event *bp)
+{
+	struct arch_hw_breakpoint *info = counter_arch_bp(bp);
+	unsigned long ibreakenable;
+
+	xtensa_wsr(info->address, SREG_IBREAKA + reg);
+	RSR(ibreakenable, SREG_IBREAKENABLE);
+	WSR(ibreakenable | (1 << reg), SREG_IBREAKENABLE);
+}
+
+static void set_dbreak_regs(int reg, struct perf_event *bp)
+{
+	struct arch_hw_breakpoint *info = counter_arch_bp(bp);
+	unsigned long dbreakc = DBREAKC_MASK_MASK & -info->len;
+
+	if (info->type & XTENSA_BREAKPOINT_LOAD)
+		dbreakc |= DBREAKC_LOAD_MASK;
+	if (info->type & XTENSA_BREAKPOINT_STORE)
+		dbreakc |= DBREAKC_STOR_MASK;
+
+	xtensa_wsr(info->address, SREG_DBREAKA + reg);
+	xtensa_wsr(dbreakc, SREG_DBREAKC + reg);
+}
+
+int arch_install_hw_breakpoint(struct perf_event *bp)
+{
+	int i;
+
+	if (counter_arch_bp(bp)->type == XTENSA_BREAKPOINT_EXECUTE) {
+		/* Breakpoint */
+		i = alloc_slot(this_cpu_ptr(bp_on_reg), XCHAL_NUM_IBREAK, bp);
+		if (i < 0)
+			return i;
+		set_ibreak_regs(i, bp);
+
+	} else {
+		/* Watchpoint */
+		i = alloc_slot(this_cpu_ptr(wp_on_reg), XCHAL_NUM_DBREAK, bp);
+		if (i < 0)
+			return i;
+		set_dbreak_regs(i, bp);
+	}
+	return 0;
+}
+
+static int free_slot(struct perf_event **slot, size_t n,
+		     struct perf_event *bp)
+{
+	size_t i;
+
+	for (i = 0; i < n; ++i) {
+		if (slot[i] == bp) {
+			slot[i] = NULL;
+			return i;
+		}
+	}
+	return -EBUSY;
+}
+
+void arch_uninstall_hw_breakpoint(struct perf_event *bp)
+{
+	struct arch_hw_breakpoint *info = counter_arch_bp(bp);
+	int i;
+
+	if (info->type == XTENSA_BREAKPOINT_EXECUTE) {
+		unsigned long ibreakenable;
+
+		/* Breakpoint */
+		i = free_slot(this_cpu_ptr(bp_on_reg), XCHAL_NUM_IBREAK, bp);
+		if (i >= 0) {
+			RSR(ibreakenable, SREG_IBREAKENABLE);
+			WSR(ibreakenable & ~(1 << i), SREG_IBREAKENABLE);
+		}
+	} else {
+		/* Watchpoint */
+		i = free_slot(this_cpu_ptr(wp_on_reg), XCHAL_NUM_DBREAK, bp);
+		if (i >= 0)
+			xtensa_wsr(0, SREG_DBREAKC + i);
+	}
+}
+
+void hw_breakpoint_pmu_read(struct perf_event *bp)
+{
+}
+
+void flush_ptrace_hw_breakpoint(struct task_struct *tsk)
+{
+	int i;
+	struct thread_struct *t = &tsk->thread;
+
+	for (i = 0; i < XCHAL_NUM_IBREAK; ++i) {
+		if (t->ptrace_bp[i]) {
+			unregister_hw_breakpoint(t->ptrace_bp[i]);
+			t->ptrace_bp[i] = NULL;
+		}
+	}
+	for (i = 0; i < XCHAL_NUM_DBREAK; ++i) {
+		if (t->ptrace_wp[i]) {
+			unregister_hw_breakpoint(t->ptrace_wp[i]);
+			t->ptrace_wp[i] = NULL;
+		}
+	}
+}
+
+/*
+ * Set ptrace breakpoint pointers to zero for this task.
+ * This is required in order to prevent child processes from unregistering
+ * breakpoints held by their parent.
+ */
+void clear_ptrace_hw_breakpoint(struct task_struct *tsk)
+{
+	memset(tsk->thread.ptrace_bp, 0, sizeof(tsk->thread.ptrace_bp));
+	memset(tsk->thread.ptrace_wp, 0, sizeof(tsk->thread.ptrace_wp));
+}
+
+void restore_dbreak(void)
+{
+	int i;
+
+	for (i = 0; i < XCHAL_NUM_DBREAK; ++i) {
+		struct perf_event *bp = this_cpu_ptr(wp_on_reg)[i];
+
+		if (bp)
+			set_dbreak_regs(i, bp);
+	}
+	clear_thread_flag(TIF_DB_DISABLED);
+}
+
+int check_hw_breakpoint(struct pt_regs *regs)
+{
+	if (regs->debugcause & BIT(DEBUGCAUSE_IBREAK_BIT)) {
+		int i;
+		struct perf_event **bp = this_cpu_ptr(bp_on_reg);
+
+		for (i = 0; i < XCHAL_NUM_IBREAK; ++i) {
+			if (bp[i] && !bp[i]->attr.disabled &&
+			    regs->pc == bp[i]->attr.bp_addr)
+				perf_bp_event(bp[i], regs);
+		}
+		return 0;
+	} else if (regs->debugcause & BIT(DEBUGCAUSE_DBREAK_BIT)) {
+		struct perf_event **bp = this_cpu_ptr(wp_on_reg);
+		int dbnum = (regs->debugcause & DEBUGCAUSE_DBNUM_MASK) >>
+			DEBUGCAUSE_DBNUM_SHIFT;
+
+		if (dbnum < XCHAL_NUM_DBREAK && bp[dbnum]) {
+			if (user_mode(regs)) {
+				perf_bp_event(bp[dbnum], regs);
+			} else {
+				set_thread_flag(TIF_DB_DISABLED);
+				xtensa_wsr(0, SREG_DBREAKC + dbnum);
+			}
+		} else {
+			WARN_ONCE(1,
+				  "Wrong/unconfigured DBNUM reported in DEBUGCAUSE: %d\n",
+				  dbnum);
+		}
+		return 0;
+	}
+	return -ENOENT;
+}